A seatbelt in a vehicle may be equipped with “load-limiting” features. During a collision, a retractor of the seatbelt may lock webbing of the seatbelt from further extension from the retractor, and load-limiting features permit some additional limited extension of the webbing when the force exerted on the webbing exceeds a load-limiting threshold. This additional extension of the webbing from the retractor limits the load applied by the webbing to the chest of an occupant to reduce the chest deflection.
Load limiting may be achieved through use of a torsion bar. As one example, in U.S. Pat. No. 7,240,924, the retractor can include a first torsion bar and a second torsion bar that are aligned coaxially. During an impact, the spool is selectively engageable with either the first torsion bar or with the second torsion bar. The second torsion bar absorbs a higher force than the first torsion bar, i.e., it takes a larger force to twist the second torsion bar compared to the first torsion bar. Accordingly, when the occupant is relatively heavy, the spool is engaged with the second torsion bar (i.e., in a high load mode), and when the occupant is relatively light, the spool is engaged with the second torsion bar (i.e., in a low load mode). The retractor includes a pyrotechnic device that engages the spool with the first torsion bar or the second torsion bar.
As another example, in U.S. Pat. No. 8,814,074, a retractor includes a torsion bar fixed to a spool. The torsion bar is selectively engageable with a deforming member, which is more easily deformable relative to the torsion bar. Accordingly, the torsion bar and the deforming member can be connected in parallel for when an occupant of the seat is relative heavy (i.e., in a high load mode), and can be connected in series when the occupant of the seat is relatively light (i.e., in a low load mode).
As another example, in U.S. Pat. No. 8,220,735, the retractor includes a torsion bar with two segments and a pretensioner activatable to switch between a high load mode and a low load mode. The first segment of torsion bar may be engaged by default, and the pretensioner may ignite a charge to engage the second segment of the torsion bar.
As another example, in U.S. Pat. No. 7,370,822, the retractor includes a brake system engageable with two torsion bars. In this situation, the brake system may be engageable with each torsion bar separately to prevent rotation of the torsion bar, i.e., to set a load limiting level. In other words, the brake system may engage both torsion bars, one of the torsion bar, or neither torsion bar to meet the load limiting requirements.